Stricter pub/sub guarantees
We've had weird pub/sub behavior reported, caused by the fact that publish is deferred until the end of the event loop iteration. This means that subscribing after a publish may very well end up acting as if you subscribed before publishing, and the opposite for unsubscribe.
In most apps this won't make a difference but now there's stricter guarantees regarding when things happen. Publish still happens at the end of the event loop iteration, but will trigger earlier if needed to keep things in proper sync.